Current industrial practice in the manufacture of zirconium and hafnium semi-finished products such as tubeshells, billets and flat channel sheets typically involves a series of high temperature steps which employ high vacuums or protective non-oxidizing atmospheres. Broadly considered, ingots are melt consolidated by either multiple vacuum arc melting or electron beam melting. The ingots are then thermomechanically processed by heating the ingots well above the alpha-beta transition temperature and high temperature forging to intermediate shapes (frequently with one or more beta quenches) that will allow further mechanical working in the alpha temperature range. Advantageously, zirconium and hafnium may be processed in the alpha temperature range in air, whereas they must be protected by a non-oxidizing atmosphere when worked in the beta range.
It has been recently proposed to melt zirconium by plasma arc processes. See, e.g., D. Apelian et al., "Electron Beam Melting v. Plasma Melting: A Critical Review", Proceedings of the Conference entitled "Electron beam Melting and Refining, state of the Art 1984", pages 18-48. This disclosure, among other things, indicates that zirconium may be plasma arc melted using carbon electrodes and that titanium scrap and sponge may be plasma arc melted to form electrodes for subsequent vacuum arc remelting. This disclosure also briefly describes a Japanese titanium slab ingot facility wherein titanium sponge is plasma arc melted in a hearth and the melted titanium then poured into a mold having a continuous withdrawal mechanism to produce slab ingots (shown in FIG. 19). A plasma melting facility for processing titanium scrap and sponge is disclosed by G. Sick, "Large Scale Plasma Melting and Remelting Tests", Proceedings of the Vacuum Metallurgy Conference, 1986, and "Plasma Melting For Titanium and Superalloys", Proceedings of the Vacuum Metallurgy Conference, 1989.
As these disclosures generally indicate, plasma arc processes employ highly capital intensive facilities and high operating costs. Thus they have not been commercially suitable in the production of zirconium and hafnium.